Broadcast television signals according to the NTSC standard in the U.S.A. occupy a video frequency band of about 4.25 MHz. Low cost home video cassette recorders are unable to record a TV signal having so wide a frequency range on magnetic tape and may be limited to recording a frequency range of about 2.5 MHz Accordingly, the definition of pictures reproduced by a video cassette recorder is considerably less than is produced by a TV set receptive of an over-the-air signal. A great deal of effort has been directed to improving the definition of pictures reproduced by a video cassette recorder, in spite of the frequency limitation of the magnetic recording process. For example, it is known to improve the definition of magnetically recorded images by folding the high frequency components, of the luminance portion of a TV signal to be recorded, into a base-band spectrum, and thereafter, unfolding and restoring the luminance high frequency components for combination with the chroma signal and reproduction in a TV set.
In processing video signals after they have been unfolded from the folded condition, the modulation carrier and the corresponding sidebands are not always removed completely. The effect of the incomplete removal of these signals is artifacts or blemishes in the final TV picture display. These artifacts are particularly evident when the object has a well-defined edge which the luminance signal (particularly in the upper bandwidths, about 4.5 MHz) can reproduce. The problem of removing these unwanted signals has heretofore been difficult to accomplish. It is important to understand the folding and unfolding processes in order to appreciate how the present invention overcomes the problem of the unwanted signals.
The folding and unfolding process involves the use of comb filters well known in the art. Thus, when an object or field of view is periodically scanned in a series of parallel lines and the light level variations are translated into electrical energy analogs, the energy is largely concentrated in a number of discrete energy groups distributed throughout the spectrum used. There is very little useful energy lying between the groups, and the spectral distance between the groups is related to the line scanning rate and the picture frame scanning rate. Most of the energy lies at the line scanning rate and harmonics thereof, and at the frame scanning rate and harmonics thereof. Comb filters are used in processing the video when folding the high frequency discrete energy groups into the spaces between the low frequency discrete energy groups, and in processing the video when unfolding the high frequency and low frequency energy groups.
A horizontal comb filter in a digitized video system includes in cascade connection two delay devices each with a delay t, where t is the delay time between picture elements (pixels), which may be the period of a f.sub.sh horizontal sampling carrier. The luminance signal before entering the first delay device, the signal leaving the first delay device, and the signal leaving the second delay device are combined in such a way as to tend to cancel the horizontal folding frequency carrier at the output of the horizontal comb filter.
Similarly, a vertical comb filter in a digitized video system includes in cascade connection two delay devices each with a delay of the delay time between successive lines, which may be the period of f.sub.sv, the vertical sampling frequency, where f.sub.sv is equal to f.sub.sh divided by pixels per line. The luminance signal before entering the first delay device, the signal leaving the first delay device, and the signal leaving the second delay device are combined in such a way as to tend to cancel the vertical component of the folding frequency and sidebands at the output of the vertical comb filter.
However, the completeness of cancellation of the horizontal frequency component, and of the vertical frequency component, is affected by the subject matter being scanned. The horizontal frequency cancellation is the best when the pixels scanned horizontally have the same brightness. And the vertical frequency cancellation is the best when the pixels scanned vertically have the same brightness. In both cases, the cancellation is the poorest when horizontally scanning a "horizontal edge" between white and black, and when vertically scanning a "vertical edge" between white and black.
According to the terminology employed herein, a horizontal scan encountering an edge between white and black is said to encounter a "horizontal edge" The edge may be a vertical line, but it is called a horizontal edge because it is encountered during a horizontal scan. Similarly, an edge encountered during a vertical scan is called a "vertical edge". The term "edge" is sometimes referred to in the art as a "discontinuity", "transition", or "detail".